The present invention relates to a process for the removal from polyether polyols of basic catalysts that remain in these products after their preparation is completed. The removal of catalyst is effected by hydrolysis and introduction of carbon dioxide, with subsequent filtering off of the carbonate formed.
Polyoxyalkylene polyether polyols, also known simply as polyether polyols, are used for the preparation of polyurethanes. The polyether polyols are most commonly reacted with polyisocyanates and give urethane polymers, which can be in the form of either elastomers or semi-rigid or rigid foams. For example, European Application 116,309, German Auslegeschrift 1,929,034, German Offenlegungsschrift 2,019,322, and Ullmanns Encyklopadie der technischen Chemie (Ullmanns Encyclopedia of Industrial Chemistry), vol. 19, page 31 et seq. Polyether polyols can also be used as textile auxiliaries, surfactants, and hydraulic fluids.
The properties of the polyurethanes depend very greatly on the polyether polyols and isocyanates used, as well as the corresponding additives, such as, for example, catalysts. It is therefore necessary for the polyether polyols to be in a very pure form and to be as free as possible from impurities that can act as catalysts in the reaction of polyether polyols and isocyanates.
Polyether polyols can be prepared on a large industrial scale by the polyaddition reaction of alkylene oxides on starter molecules with active hydrogen atoms. For example, Ullmanns Encyklopadie der technischen Chemie (Ullmanns Encyclopedia of Industrial Chemistry), vol. 19, page 31 et seq. Polyether polyols having free hydroxyl groups are obtained, although terminal alcoholate groups can be present because of the alkaline reaction medium. Alcoholate groups can be converted into free hydroxyl groups in a subsequent step. The alkali-containing polyether polyol preparations are generally neutralized with inorganic or organic acids to produce polyether polyols containing hydroxyl groups and aqueous salt solutions. The water is then removed by distillation and the salts are separated off from the polyether polyols by filtration. Alternatively, the polyether polyol can be separated by centrifugation and/or phase separation, either with or without the addition of inert solvents.
If inorganic acids, such as sulfuric acid, phosphoric acid, hydrochloric acid, potassium hydrogen phosphate, and the like, or organic acids, such as citric acid, tartaric acid, and the like, are used for the neutralization, it is necessary to carry out the neutralization up to the exact equivalence point. This precision is intended to assure, on the one hand, a minimum of residual basic alkali salts and, on the other hand, a minimum of excess acid. Furthermore, the alkali salt is often precipitated in such a fine form that filtration is difficult despite the use of filtration aids. The use of strong acids can also lead to side reactions, such as esterification, etherification, and/or dehydration of terminal hydroxyl groups, and/or to the degradation of the polyether chains. Polyethers damaged in this way can contain unpleasant odor components.
Another possible method for removal of the catalyst is hydrolysis of the alcoholate and subsequent phase separation using inert organic solvents. See U.S. Pat. No. 3,715,402. The phase separation can be aided by centrifugation or electrostatic coalescence, but such variants on the process are limited to water-insoluble polyether polyols.
The use of magnesium silicates as absorbents for the removal of aqueous potassium hydroxide is described in a number of patents, for example, German Offenlegungsschrift 2,208,614 and U.S. Pat. Nos. 4,029,879 and 4,137,396. The filtration residue thereby obtained, which consists of potassium hydroxide-magnesium silicate and polyether polyol, is difficult to handle and too problematical to dispose of. Losses in yield are, moreover, relatively high.
U.S. Pat. No. 3,833,669 teaches inter alia that, for the removal of catalysts by neutralization with carbon dioxide, the catalyst is inadequately neutralized and the very fine crystals of alkali metal carbonates are difficultly filtered. Consequently, polyether polyols not having the desired degree of purity are obtained. The formation of sparingly soluble alkali metal-magnesium carbonate double salts is recommended in this patent. The disadvantages of this process include the handling and disposal of the filtration residue. Moreover, the use of large stoichiometric excesses of carbon dioxide (two- to ten-fold, plus waste gas disposal) and magnesium salts (one to 20 times the catalyst content, plus waste disposal) is unsatisfactory. In addition, 0.1 to 1% of the amount of polyether polyol is required when separating off the filtration aids, which additionally makes any possible later use of the residues more difficult.
The object of the present invention was therefore to provide a simple and reliable process for the removal of catalyst residues from polyether polyols which does not have the disadvantages discussed above.